For preparing 3-piperazinylbenzisothiazole derivatives, mainly two processes have been hitherto known. One of them is a process by reacting 3-halo-1,2-benzisothiazole with a piperazine compound according to the following reaction scheme (1): ##STR1## (JP-A 63-83067; JP-A 63-83085; EP-A 196096; J. Chem. Soc., Perkin. Trans., 1(8), 2141. 1988; Ger. Offen., 3530089; J. Med. Chem., 29(3), 359, 1986), and the other one is a process which is disclosed in JP-A 8-291134 which was previously filed by the present applicant, according to the reaction scheme (2): ##STR2## wherein X represents Cl or Br, and R represents H, alkyl group having 1 to 6 carbon atoms or substituted alkylene group having 1 to 6 carbon atoms.